Raised Garden Soils 101:
The type of garden soils that you choose to use in your raised flower beds are essential to how well your plants will grow. Unfortunately, most of us were not blessed with perfect soil for starting a plant or vegetable garden, so we have to either choose our plants carefully, or we have to add amendments to our garden soil to make it more desirable.
There are 3 basic criteria that any gardener looks for in a soil, whether it's the stuff Mother Nature provided, or it's your own mix of garden soils:
The soil should be firm enough to support the plants. If the soil is too loose, the plants are likely to fall or get blown over from their own weight. The soil should have good moisture retention, which also means it will retain nutrients as well. If the water and nutrients just drain through to the sub-soil, then the plants will not do well. Your raised garden soils should also be loose enough to allow for good aeration and drainage. Soil that is too thick or compacted will smother the roots of plants rather than let them breath properly.
Ideal Raised Garden Soils:
The best raised garden soils you could hope for would be a form of loam soil. Loam soils are the holy grail of gardeners everywhere because it will grow just about anything with terrific results. Loam soil is the proper mixture of clay particles, sand particles, and organic matter, all balanced perfectly. What makes loam soil so desirable is that it meets or exceeds all the basic requirements needed to grow plants and vegetables to maximum fruition.
While naturally occurring loam soil is a very rare find, there are various ways that one can try to duplicate the mixture and create your own garden soils. The best "base" to start with is a nice rich compost soil that you've created using a good mix of "browns" and "greens" for the right nutrients.
From there it gets a little tricky as there are a lot of different amendments that you can add to the mix, with each one serving a particular function, but in some cases causing its own problem as well.
Amending Raised Garden Soils:
For instance you will need to blend roughly 25% - 30% sand to the mix to help keep it loose. Do not overdose the sand in your mix or else it will be too thin and crumbly and will not hold up. You also want to make sure you are mixing your garden soils with river sand and not beach sand from the ocean. Ocean sand has a very high salt content and you will find that few plants are tolerant of the salt content in their soil.
The next consideration you need to think about is moisture retention. Moisture retention equals nutrient retention and that means healthy plants. There is a fine line between having proper moisture retention, and having garden soils that drain too rapidly. One of the most commonly used items for moisture retention is peat moss because it absorbs and holds moisture well. You can also use aged bark that is ground fine, and even sawdust works well. You are going to want at least 50% of your mix to be of this material.
That leaves you with 20% - 25% left for adjustments. Many people will add the first two items to their existing soil and till them altogether. This is perfectly ok provided your native soil is not too heavy or too light in sand particles. If you are fortunate enough to have soil that is a little heavier in clay deposits to be used by itself, that will make a great base for your raised garden soils.
Keep Your Raised Garden Soils Balanced:
You will need to be careful when adding amendments like fresh compost and sawdust, as they will change the composition of the soil and may need other amendments added. Fresh organics for example will leach a lot of nitrogen during the first season or two, so you may need to replenish that by using a nitrogen fertilizer.
Vermiculite and perlite are inorganic additives that you can use in your raised garden mixes in place of sand. When adding amendments, you need to consider their permeability, as well as their ability to retain water.
Vermiculite has a high permeability, as well as high water retention properties. Vermiculite can be found at any local garden center for a reasonable price. Vermiculite has a great many uses, one of which is it is used to absorb hazardous spills because of its ability to retain liquids. This is a great additive to use if your soil is heavy in clay.
Perlite is another common inorganic used in raised garden soils. It also has a high permeability, but very low water retention. This is a good additive to use if you have a high compost or peat content in your soil. It is very common for gardeners to use a mix of 50% compost and 50% perlite mix for their gardens. Compost tends to have a low-medium permeability but a medium-high retention capability, so the perlite is the right additive to balance out both of those factors.
Other Amendments for Raised Garden Soils:
Wood chips, sawdust, and bark are commonly used as we've said before, however you need to keep in mind that these wood products have a high permeability, and a low-medium water retention capability. This will help you out if your soil does not drain properly, but if moisture retention is what you need, you should switch to a humus type additive for your raised garden soils.
Sand particles will have a high permeability but low water retention. You will want to use this as an additive sparingly or else you will risk having your garden soil drain too quickly.
Silt and clay on the other hand have very low permeability, but very high water retention, so they make a great additive for soils that drain too quickly.
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